During the drilling of subterranean wellbores, the upper portion of the drill string is composed of drill pipe which is operated under tension, the lower portion of the drill string is generally composed of drill collars whose weight is applied to the drill bit thereby placing the collars in compression, and torque is applied to the entire drill string in order to rotate the drill bit. Accordingly, the weight of the drill collars in compression can be resolved into two component forces: a first acting parallel to the axis of the collars and a second acting normal to the first, perpendicular to the wellbore. Similarly, the load applied by the drill bit to the rock can be resolved into axial and normal component forces. The loads transmitted to the drill bit by the lowermost collars and the drill string affect the magnitude and direction of the forces applied by the drill bit to the rock.
If during drilling the magnitude and direction of the normal forces in the vicinity of the drill bit can be controlled, then theoretically the bit can be steered and caused to drill in any desired subsurface location.
Various processes and associated equipment have been developed to both enhance and minimize this normal force. In regard to minimizing the normal force, various types of drill collars, stabilizers and the like, has been used to continuously restore the borehole to vertical. For example, U.S. Pat. No. 4,319,649 discloses drill collar stabilizers to maintain the drill string in a vertical direction. In U.S. Pat. No. 4,220,213, there is described placing an eccentric member having a thickwalled side and a lighter side concentrically about a drill collar to maintain drilling in the vertical direction. In this method, gravity will cause the thick-walled portion of the eccentric collar to rotate to the low side of the deviated drill string.
Various types of drill collar stabilizers have also been used to alter the direction of drilling. U.S. Pat. Nos. 4,305,474 and 4,465,147 disclose stabilizers that create a force perpendicular to the drill string in order to steer the drill bit along a desired direction.
One type of drilling process that is particularly difficult to control is called horizontal drilling. Horizontal drilling involves creating a highly curved wellbore having a tight radius and curvature. This type of drilling can be characterized by a rate of change of angle on the order of 0.5-6.degree./ft, resulting in radii of curvature of 10-120 ft and a final drift angle of about 90.degree..
In U.S. Pat. No. 4,699,224, there is disclosed a method of horizontal drilling using a flexible drill string connected by a flexible joint to a drill bit collar equipped with a stabilizer, and rotary drill bit. This method includes the use of an eccentric cylindrical collar having a sidewall engaging means that circumferentially mounts to the downhole end of the flexible drill string directly over the flexible joint leading to the drill bit collar. This eccentric collar forces the drill string passing therethrough to one side of the wellbore, thus lever arming the drill bit to the other side of the wellbore by pivoting on a stabilizer mounted to the drill bit collar between the flexible joint and the drill bit.
Although with the aid of the borehole engaging means the cylindrical eccentric collar is designed not to rotate with the drill string, friction between the collar and drill string plus downhole vibrations occurring during drilling will tend to rotate the collar, thereby resulting in the need for the collar to be reoriented periodically while drilling the curve from vertical to horizontal.
One alternative would be to mark the pipe as it is being introduced into the wellbore, and once in the wellbore, rotate the drill string counterclockwise, stopping at the correct orientation, the surface pipe mark. Although this method can be used in shallow wells, an alternative method is needed to orient the collar in deeper wells due to the unpredictable twist of the drillstring.
One method of orienting the eccentric collar would be to use a steering tool as disclosed in Marshall, Survey Steering Tool: The Ultimate for Saving Rig Time, SPE 5897 (1976), but this would be expensive because it would require a wireline truck and associated equipment.
Another method would be to use a single-shot orientation survey, for example, the device described in U.S. Pat. No. 4,171,578. The disadvantage of this method is the time required to introduce the instrument into the wellbore for each survey.
The accuracy of both the steering tool and the single-shot survey methods would be limited by the torsional flexibility of the drill collars since the orientation sub would have to be located above the drill collars because the internal diameter of the collars is not sufficient for the survey tool to pass through.
In U.S. Pat. No. 3,983,948, there is disclosed a tool for indicating the orientation of a downhole drilling assembly. This tool is undesirable because it is complicated with many intricate parts susceptible to breakdown in the harsh borehole environment. Further, the tool is not amenable to compact packaging for use in small diameter wellbores, for example, of less than 6 in. in diameter. Therefore, the tool is not appropriate for high curvature or horizontal holes. Moreover, this tool appears to be limited to drilling with a downhole motor.
There is a need for a simple device with minimal moving parts for orienting a downhole drilling assembly used in building angle during horizontal drilling. There is a novel need for a compact device for use in orienting downhole drilling assemblies, including a steering tool assembly that anticipates use in boreholes that have high curvature.